PROCESS FOR THE PREPARATION OF FULLERENES

Abstract: A process for the preparation of fullerenes comprises subjecting one or more optionally substituted aromatic hydrocarbons to pyrolysis.

Description: Description Process for the Preparation of Fullerenes The present invention relates to a novel process for the preparation of fullerenes. The process which currently forms the basis of commercial fullerene production (W. Kratschmer et al, Nature, 347, pp 354 - 358, 1991) involves the arc vaporisation of graphite rods in a helium atmosphere at about 15 % normal atmospheric pressure. After purification the yield of fullerenes is approximately 5 %. Although the known process enables the production of fullerenes in relatively large quantities, the method does have a number of disadvantages. The process is essentially a batch process since the graphite rods (which are expensive) are consumed in the process. Although various techniques have been employed to try to increase the production per batch such techniques tend to be complex and costly. In addition the reaction produces a large quantity of an extremely light and static soot. Removal of the soot from the reaction system is a difficult task and is potentially hazardous since the by-products formed and perhaps even the fullerenes themselves may be carcinogenic. Other methods for the preparation of fullerenes have been devised but they all suffer from disadvantages; they are essentially batch processes and/or they are very costly or result in large amounts of by-products. We have now developed a new process for the preparation of fullerenes which has advantages over the standard production method. Thus, according to one aspect of the present invention we provide a process for the preparation of fullerenes which comprises subjecting one or more optionally substituted aromatic hydrocarbons to pyrolysis. Whilst the presence of relatively small concentrations of oxygen may not be detrimental it is desirable to takes steps to minimize the oxygen content of the pyrolysis reaction environment. This may, for example, be achieved by using an inert gas atmosphere, for example helium, argon or nitrogen or, alternatively, the pyrolysis may be performed in vacuo. The process according to the present invention has the advantages that it enables inexpensive materials to be used as the feedstock, can be performed using simple apparatus and may be adapted to provide a continous production process. The reaction products may also be recovered in a less hazardous manner. As aromatic hydrocarbons, polycyciic aromatic hydrocarbons are preferred, in particular those having 10 - 32 carbon atoms for example anthracene, phenanthrene, fluoranthene, ovalene, corannulene and naphthalene, naphthalene being especially preferred. The aromatic hydrocarbons may be substituted partially or totally by leaving groups like halogen, in particular chlorene; NHR, NR 2 or OR with R = H, C^C^-alkyl or acyl; OS0 2 -R 1 with R 1 = C j -C^alkyl or C 6 -C 10 -aryl; or C 1 -C 4 -alkyl, preferably methyl or ethyl. The pyrolysis is conveniently performed in the gas phase at a temperature between 500 ° and 3000 °C, preferably between 800 ° and 1500 °C, most preferably around 1000 °C. A preferred method for performing the process according to the present invention is to pass the vaporised aromatic feedstock material through a tube heated to the appropriate temperature. The tube may be made of any suitable material such as, for example, silica (for temperatures up to approximately 1300 °C), a metal, for example iron, steel, nickel and copper (up to temperatures just below their melting points) or ceramic materials. The rate of evaporation of aromatic feedstock is desirably kept below 600 mg min "1 ; when using naphthalene a rate of evaporation of around 200 mg min '1 is particularly preferred. When the process is performed in a stream of inert gas flowing through a heated tube, the flow rate for a 1 cm diameter tube is typically up to 50 ml min '1 for a heated region up to 5 cm in length. With larger tubes and/or longer or multiple heated regions appropriate adjustments to the flow rate may be made. The process according to present invention may be assisted by the use of a suitable catalyst, preferably a metal or metal containing catalyst, for example, nickel powder, palladium on carbon or metallic or non-metallic oxides. The fullerenes produced are subsequently isolated from the effluent gases from the pyrolysis reaction. The preferred method for achieving this is firstly to pass the effluent gas into a cooling system, for example a cooling core where the majority of the pyrolysis product will collect. Cooling may be achieved by any suitable coolant, for example water, ice or dry-ice. The gas stream is then passed through one or more solvent scrubbers, the pyrolysis reaction tube and cooling core are washed out, the resulting solvent slurry of the products is filtered and the filter cake is thoroughly washed. The solvent used in the scrubbers and for the subsequent washing steps is conveniently one in which the unwanted by-products dissolve, for example a ketone, especially acetone, or an ether, especially diethyl ether. The filter cake is then washed with a suitable solvent in which fullerenes are soluble, for example carbon disulfide, toluene, benzene, xylenes or 1,2-dichlorobenzene. The resulting solution may then be concentrated and purified by the standard methods for the purification of fullerenes. Separation of the various fullerenes produced may then be achieved by standard chromatographic methods. Examples of suitable methods include those devised by R. Taylor et al (J. Chem. Soc, Chem. Comm., pp 1423 - 1425, 1990). The by-products obtained in the process which are soluble in the polar solvent used include fullerene precursors and these may be recycled to increase the yield of the process. Such recycling of precursors allows for a continous preparation process. Using for example naphthalene as the aromatic hydrocarbon the principal fullerenes produced in the process are Cgg and C 70 . However, use of appropriate aromatic hydrocarbons as feedstock, either individually or in combination, may result in production of other fullerenes. The process according to the present invention may also be useful in the production of endohedral fullerene metal complexes. The standard synthesis of endohedral fullerene complexes entails a slight modification of the basic Kratschmer method i. e. resistive vaporisation of graphite impregnated with metal oxides or graphite rods into which metal rods have been drilled. The yield of endohedral complexes from such a process is, however, very low. The process of the present invention may be modified by passing the vaporised aromatic feedstock over a metal source heated to an appropriate temperature resulting in the production of endohedral fullerene complexes containing the metal of the source. Metal sources used for the endohedral fullerene metal complex are for example sole metals, metal halides, metal oxides and/or metal carbonates. Although the precise mechanism of the production of fullerenes according to the process of the present invention is not known, analysis of the intermediate products by suitable spectroscopic techniques suggest that carbon fragments are condensed in a stepwise manner leading to various polycyclic aromatic hydrocarbon frameworks which undergo elimination and finally collapse to fullerene cage structures. The process according to the present invention is further illustrated by the following non-limiting example. Example 1 A deposit of naphthalene was placed at the inlet end of a 1 cm diameter Pyrex ® glass tube of 40 cm length. Argon was passed through the apparatus at the rate of 15 ml min '1 for a few minutes, the naphthalene was gently heated with a butane torch and a 2 cm length near the outlet end of the tube was heated with a propane/oxygen torch to approximately 1000 °C. As the naphthalene vapour passed over the heated region pyrolysis occurred. The effluent gases were passed through an outlet of the tube into a glass coil immersed in dry ice and then into a triple batch of acetone scrubbers. The contents of the tube and cooling coil were washed out with acetone and combined with the acetone from the scrubbers. The acetone slurry of the products were filtered and washed thoroughly with acetone to leave a black sooty deposit. This was then washed with carbon disulfide to give a deep red solution. The solution was concentrated and purified by standard techniques to produce Cg Q and C 70 , the combined yield of Cg Q and C 70 being approximately 0,5 %. The mass spectrum under E. I. conditions of the concentrated CS 2 solution is illustrated in figures 1a and 1b and shows the following main features: i) m/z = 376/374. Evidence of a trinaphthyl species further condensed to form a naphthylbenzofluoranthene or a naphthoperylene system with elimination of eight or ten hydrogens. ii) m/z = 500/498/496. Evidence of a species formed from fusion of two benzofluoranthenes with up to two additional ring fusions and elimination of up to sixteen hydrogens. iii) m/z = 624/622/620. Evidence of further fusion of e. g. benzofluoranthenes with naphthalene followed by two further ring closures and elimination of up to twenty hydrogens. These species, which must be partly curved, now show the characteristic C 2 fragmentation loss. iv) m/z = 752/750/748/746. Evidenve of e. g. tetranaphthylbenzofluoranthene with further ring closures and elimination of up to twenty-two hydrogens. v) m/z = 720. This is clearly Cgg, showing the characteristic lack of m-2 fragments. vi) m/z = 840. This is clearly C 70 .

Claims: Patent claims 1. A process for the preparation of fullerenes which comprises subjecting one or more optionally substituted aromatic hydrocarbons to pyrolysis. 2. The process according to ciaim 1 characterized in that the pyrolysis is performed at a temperature between 500 and 3000 °C. 3. The process according to claim 1 and/or 2 characterized in that the pyrolysis is performed in an inert gas atmosphere or in vacuo. 4. The process according to one or more of the claims 1 - 3 characterized in that the vaporised aromatic feedstock material is passed through a tube heated to the appropriate temperature. 5. The process according to one or more of the claims 1 - 4 characterized in that the rate of evaporation of the aromatic feedstock is kept below 600 mg/min. 6. The process according to one or more of the claims 1 - 5 characterized in that the process is additionally assisted by the use of a suitable catalyst. 7. The process according to one or more of the claims 1 - 6, characterized in that the fullerenes produced are subsequently isolated from the effluent gases from the pyrolysis reaction by firstly passing the effluent gases into a cooling system, then passing the gas stream through one or more solvent scrubbers, washing out the pyrolysis reaction tube and cooling core, filtering the resulting solvent slurry of the products and washing the filter cake with a suitable solvent in which fullerenes are soluble, optionally concentrating and purifying in the resulting solution and optionally separating the various fullerenes produced. 8. The process according to one or more of the claims 1 - 7, characterized in that the aromatic hydrocarbon is polycyclic. 9. The process according to claim 8, characterized in that the polycyclic hydrocarbon is naphthalene. 10. Use of the process according to claim 1 for the production of endohedral fullerenes.

Inventors: TAYLOR ROGER, TAYLOR, ROGER
Applicants: HOECHST AG, TAYLOR ROGER, HOECHST AKTIENGESELLSCHAFT, TAYLOR, ROGER
Application year: 1994
Main classification: C01B31
IPC codes: C01B31